Axle load to load index

Three inputs. Axle load in kilograms — normally the plated axle weight, or the worst-case figure from a weighbridge if you have one. Tyres on the axle — two for a single fitment, four for a twin fitment, six or eight for heavy plant and multi-wheel positions. Safety margin — a percentage added to the per-tyre load before the lookup, defaulting to 10%.

The calculator divides the axle load by the number of tyres, adds the margin, then walks up the ISO load index table until it finds the first index that meets or beats that figure. It returns that index, the load it carries, and how much headroom is left over.

The margin is a judgement, not a standard. Ten per cent is offered as a default because plated weights are rarely the worst case, load is rarely shared evenly, and the penalty for a tyre one index short is far higher than the cost of one index spare. Set it to zero if you want the bare arithmetic.

A drive axle plated at 11,500 kg, twin fitment, so four tyres, with the default 10% margin.

• 11,500 kg over four tyres is 2,875 kg per tyre
• Plus 10% margin: 3,163 kg per tyre required
• The first index that covers it is 149, at 3,250 kg per tyre
• Headroom at 149: 87 kg per tyre

Note what the margin did. Without it the requirement is 2,875 kg per tyre, which index 145 covers at 2,900 kg. Adding 10% takes the requirement to 3,163 kg, and index 148 at 3,150 kg then falls 13 kg short and is rejected. Four indices, decided entirely by a margin you chose. That near miss at 148 is exactly the case the margin exists to catch.

Now take it back to the sidewall. On a twin fitment it is the second index that has to meet the number, not the first. A tyre marked 315/80R22.5 156/150L offers 150 in dual fitment, which is 3,350 kg — clear of the 3,163 kg needed. A tyre marked 154/150M would also clear it. One marked 154/148L would not.

The lookup itself is exact — it uses the same ISO table as the load index calculator. The assumptions are all in the model of the axle, and there are four worth knowing:

• Even sharing. The calculator divides the load equally between every tyre on the axle. Real axles do not behave that way. Road camber, cornering, off-centre loading, a pressure difference between twins and any diameter mismatch in a twinned pair all put more load on one tyre than another.
• Static load. It uses a standing weight. It does not model weight transfer under braking, on ramps, on site articulation, or the load a tail-lift or crane puts through one end of the vehicle while it is working.
• Correct inflation. The index is the load at the tyre’s marked pressure. Below that pressure the tyre carries less than the table says, and the margin you set is quietly spent on under-inflation.
• Normal road service. No speed derating, no agricultural cyclic field work bonus, no IF or VF allowance, no OTR duty cycle. Those all have their own tables.

The calculator does not check product availability. It will tell you that you need index 173, but not whether anything in your fitment is made at 173.

It gives you a minimum, not a specification. Fit that index or higher, then check the other three limits that the calculator cannot see: the axle’s own rating, the rim’s rating, and the vehicle’s plated weights. The lowest of the four governs, and it is not always the tyre.

Check the vehicle manufacturer’s approved tyre specification and plated axle requirements. Weighbridge data helps assess actual loading, but it does not replace these requirements or permit any vehicle, axle, wheel or tyre limit to be exceeded.

Resist the urge to over-specify by four or five indices as a matter of course. A higher index generally means a stiffer casing run at a higher pressure. On road that costs ride quality and, on some patterns, irregular wear if the tyre never sees the load it was built for. In the field it costs compaction, which is the whole reason agricultural fitments moved the other way. Choose the index the axle needs, plus a margin you have thought about.